Originally Posted by Fudgie
Because they think the corruption is spread so badly that they were able to infiltrate every level of the city's infrastructure because of it. They think it's beyond saving. They think the only way it can be truly stopped is to kill them. It's crazy but you can see why it makes sense to them.
That still doesn't quite hold up to scrutiny though. The city is corrupt because many officials are on the mob's payroll. Eliminate the mob (and maybe a few corrupt judges and cops too, to scare people) and that problem is gone and theoretically the city could go back to rebuilding itself. The leap from wanting crime gone to destroying an entire city is massive and begs the question of what the deeper ideals of the LOS are all about. Which they talk about in Batman Begins. You don't burn the world's greatest city to the ground because you want to stop crime, you do it because you want to hit the reset button on civilization. Which is also why one would try to economically attack a city (like they did prior to BB).
BTW that idea about Batman getting a sex change is BRILLIANT. Holy crap, I actually dislike the movie more now that I see what a ballsy and provocative ending Nolan missed out on!